In general, a color cathode ray tube is provided with a vacuum envelope including a substantially rectangular panel and a funnel. A phosphor screen is formed on the inner surface of an effective portion of the panel. A substantially rectangular shadow mask is provided in the vacuum envelope, facing the phosphor screen.
In the neck of the funnel, an electron gun which emits electron beams is provided. Further, in the color cathode ray tube, three electron beams emitted from the electron gun are deflected by a deflection yoke mounted on the outside of the funnel, and scan the phosphor screen horizontally and vertically through electron beam passage apertures of the shadow mask, thereby displaying a color image. At this time, the apertures of the shadow mask select and allow the three electron beams to land on desired ones of three color phosphor layers which construct the phosphor screen.
The shapes of the electron beam passage aperture can be roughly divided into two types, i.e., a circular shape and a rectangular shape. Display tubes which display text and figures mainly use a shadow mask having circular apertures. Home-use picture tubes used generally at home mainly have a shadow mask including rectangular apertures. In any case, each of the apertures is basically defined by a through hole which includes a large hole opened in the surface of the shadow mask facing the phosphor screen and a small hole opened in the surface facing the electron gun. The large and small holes are connected with each other.
An important characteristic of this kind of color cathode ray tube will be luminance of the screen. In order to improve the luminance of the color cathode ray tube, various techniques have conventionally been discussed. Techniques which have been taken over today will be adoption of a metal back layer provided on the surface of the phosphor screen facing the electron gun, use of various high-luminance phosphor materials, and the like.
In recent years, there is a known method in which the luminance is improved by increasing the high voltage called Eb of the color cathode ray tube to respond to large screens. This Eb is a voltage which is applied to the phosphor screen, shadow mask, and inner surface of the funnel of the color cathode ray tube. By increasing Eb, the speed of the electron beams is increased so that energy of collision to the phosphor material can be increased. As a result of this, the luminance based on the phosphor material is improved.
In case of increasing Eb, however, the passing time of the electron beams which penetrate through a magnetic filed generated by the deflection yoke is shortened, and accordingly, the deflection range of the electron beams is reduced. Consequently, in this case, the deflection power must be increased undesirably from the viewpoint of energy saving.
Further, improvements in luminance by a method called a focus mask have conventionally been tried although the method has not yet been put into practice. In the following, explanation will be made of principles of the focus mask.
A color cathode ray tube which is considered as a main trend today comprises internally a shadow mask which functions as a color selection electrode, as described above. Further, electron beams emitted from an electron gun are subjected to scanning by a deflection yoke. Thereafter, the electron beams partially pass through apertures of the shadow mask and collide into the phosphor surface. At this time, about 20% of total electron beams emitted from the electron gun passes through the apertures of the shadow mask. The other remaining portion of about 80% merely collides into the shadow mask but does not contribute to the luminance of the screen. The focus mask has an object of making the electron beams which thus collide into the shadow mask reach the phosphor surface.
More specifically, in case of a focus mask, electrodes are provided on the surface of the shadow mask on the side facing the electron gun. A different potential from that to the shadow mask is applied to these electrodes, and a four-pole lens is constructed by the shadow mask and the electrodes. The four-pole lens changes the path of the electron beams to guide the electron beams to the phosphor surface.
For example, as disclosed in Japanese Patent Application KOKAI Publications No. 52-87970, No. 52-87972, No. 52-89068, and No. 56-3951, and U.S. Pat. No. 4,427,918, there has been proposals for a structure in which an insulating layer is provided on the side of the shadow mask facing the electron gun and electrodes are formed on the insulating layer. The manufacturing method thereof is disclosed in Japanese Patent Application KOKAI Publication No. 63-62129 and the like.
However, in the structure shown in Japanese Patent Application KOKAI Publications No. 52-87970, No. 52-87972, No. 52-89068, and No. 56-3951, both electrodes are made of metal plates. It is difficult to position precisely those two electrodes over the entire area of the screen.
In another structure according to the known publications as described above, two bamboo-blind-like electrodes are arranged to be perpendicular to each other, thereby to form apertures of the shadow mask. In this structure, however, it is difficult to form the curved surface of the shadow mask. At the same time, the apertures of the shadow mask cannot substantially be arranged in a stagger array in which the apertures are shifted at ½ pitch in the longitudinal direction of the apertures. If the apertures cannot be in a stagger array, interference fringes called moiré appear on the screen, so that the display quality of the screen is greatly degraded, leading to poor realization.
Further, in the structure disclosed in the U.S. Pat. No. 4,427,918, there is formed a part called a ridge in which the height of a non-hole part extending in the column direction of the apertures of the shadow mask is arranged to be greater than that of the other part. An electrode is formed thereon. In this structure, it is substantially impossible to change partially the shape of the electrode. It is therefore difficult to vary the electrode layout between the screen center part and the peripheral part of the screen. If this structure is used in a color cathode ray tube, it cannot be considered that an excellent convergence effect on electron beams is obtained over the entire screen.
Also, this structure is the same as that in the case of using a shadow mask material having a thicker plate thickness than that of a conventional structure. There is a risk that a part of the electron beams deflected toward the peripheral region of the screen collides into the ridge part of the shadow mask. In this case, a shadow generally called an eclipse appears. It is hence estimated that improvements of the luminance at the periphery of the screen are degraded.
As has been described above, in case of a focus mask which has conventionally been proposed, high precision is required in formation of electrodes and it is difficult to form the shadow mask surface in a desired shape. In addition, there is a problem that the degree of freedom is low in formation of electrodes, so that control of electron beams is substantially impossible.